1. Field of the Invention
The present invention relates to an air fuel ratio detecting apparatus.
2. Description of Related Art
Similar air fuel ratio detecting apparatus are disclosed in Japanese Examined Patent Publication (Kokoku) No. 63-39852 and Japanese Patent Application Laid-Open (Kokai) No. 61-186849.
In the case of Japanese Examined Patent Publication No. 63-34852, a pair of electrodes are formed in a solid electrolyte made of oxygen ion conductive metal oxide, and a working electrode, which is to be exposed in a gas to be measured is covered with a gas diffusion resistance layer.
In the case of Japanese Patent Application Laid-Open No. 61-186849, a wall member is juxtaposed in parallel with spaces relative to a working electrode, which is to be exposed in the gas to be measured, of a pair of electrodes formed in solid electrolyte. A diffusion chamber into which the gas flow is restricted is formed between the wall member and the solid electrolyte.
In the former case, a diameter of fine pores in a gas diffusion resistance layer is a primary factor for determining the diffusion resistance limited to the oxygen contained in the gas to be measured. On the other hand, in the latter case, a gap dimension between the wall member and the solid electrolyte, and a distance from a tip end of a diffusion chamber to a surface of an end of the working electrode, are factors for determining the diffusion resistance restricted to the oxygen contained in the gas to be measured.
These air fuel ratio detecting sensors are used to forcibly pump oxygen contained in the gas to be measured from one electrode to the other electrode of the solid electrolyte by applying a constant voltage between the pair of electrodes, and to measure a limit current that occurs between electrodes under this condition.
The limit current is adversely affected by the diffusion resistance of the gas to be measured in the diffusion resistance layer or the diffusion chamber.
The conventional air fuel ratio detecting apparatus suffers from the following disadvantages. Namely, the former case suffers from the following disadvantage in the manufacturing quality of the gas diffusion resistance layer, making it difficult to obtain a stable and good characteristic. In a first method of manufacturing the gas diffusion resistance layer, ceramic power such as spinel is adhered in a plurality of layers by plasma welding. As shown in FIG. 9, in the gas diffusion resistance layer 91 made according to this method, an adhesion thickness .delta. is not constant and diameters of fine pores 911 for gas diffusion are not constant.
FIG. 9 shows a slight modification of the cell 90 of the conventional air fuel ratio detecting sensor according to the former case. Reference numeral 92 denotes a solid electrolyte, numeral 93 denotes a working electrode, numeral 94 denotes a reference electrode, reference numerals 95 and 951 denote an ambient air introduction duct and its vent hole, respectively, and numerals 96 and 961 denote an electric thermal heater and its heating elements respectively.
In FIG. 9, fine pores 911 are depicted like tunnel-shaped passages. However, the actual fine pores 911 are gaps formed between the ceramic particles 910 which form the diffusion resistance layer 91, as shown in FIG. 10. The exhaust gas 8 is diffused and permeated as indicated by the arrows in FIG. 10.
As described above, the thickness .delta. of the gas diffusion resistance layer 91 manufactured by plasma welding is not uniform, and in particular, the diameter of the fine pores varies remarkably. Accordingly, the diffusion resistance changes depending upon a position of the gas diffusion resistance layer and the amount of gas to be measured (exhaust gas) passing through the fine pores varies.
As a result, the limit current of the oxygen ion current will hardly be settled at a constant level to cause a detection error as shown by a curve in FIG. 7.
In order to improve such a defect, for example, it is necessary to make uniform and fine the diameter of the fine pores of the gas diffusion resistance layer 91. However, it is practically a manufacturing impossibility to satisfy this requirement without considerably increasing manufacturing time and cost. Also, it is possible to consider that the thickness .delta. of the gas diffusion resistance layer 91 imparts the diffusion resistance. However, in this case, an extremely large thickness is required, so that the diffusion resistance layer itself is large in size and overall cell is enlarged. Also, if the diffusion resistance layer is large in size, the heat capacitance of the resistance layer is enlarged. Accordingly, it is likely that the heat of the electric thermal heater 96 would be used mainly for the diffusion resistance layer, as a result of which it would be impossible to effectively heat the solid electrolyte 92 and low temperature activation of the solid electrolyte 92 by the heater 96 would be difficult to obtain.
The latter case has the structure as shown in FIG. 11. In the latter case, in order to obtain a good limit current characteristic, it is necessary to keep the gap d between the wall member 97 and the solid electrolyte 92 exactly in parallel. This requirement is difficult to satisfy, due to the manner in which the units is desired. The unit includes the wall member 97 that is held to the solid electrolyte in a cantilever manner.
On the other hand, it is possible to considerably increase the length L from the tip end of the diffusion chamber 98 to the end face of the working electrode 93. However, this enlarges the structure corresponding to the solid electrolyte.